The typical coolant used in DIY setups is just distilled water... which is fairly non-conductive until it either picks up enough contaminants, encounters a high enough voltage, seeps into an area where there's only a tiny gap between contact points, or a combination of the three. Many people will add dyes and stuff (like corrosion inhibitors, if they're mixing incompatible metals) to their coolant which increases its electrical conductivity... but the general rule is don't build a system that leaks.

There are pre-mixed "non-conductive" coolants available but they're all water based and, arguably, aren't any safer than distilled water. Very few ever venture into experimenting with truly non-conductive coolants like Fluorinert.